Like other female athletes, the physical demands she placed on her body over a lifetime of sports and training for two marathons had taken its toll. By 2009, she had dropped 10 pounds off her 5-foot, 1-inch frame.

"A lot of them are runners and ballet dancers because they starve themselves, and gymnasts with no body fat -- anyone who takes anything to the extreme," he said.

Hoffman knows firsthand. He treated nine-time swimming medalist and Olympic swimmer Dara Torres when she was trying to conceive. She made a dramatic comeback at the age of 41 at the 2008 Games in Beijing.

Torres became a hero to millions of American women when she spoke out about the trouble she had conceiving her now 4-year-old daughter, Tessa Grace, the daughter she shares with Hoffman. [Hoffman did not treat Torres for her successful pregnancy.]

Reproductive specialists say about 12 percent of those infertile women are athletes, and the problem is becoming more acute now that more women are testing the limits of their body -- and their biological clock.

Fertility Problems Come From Over-Exercise

"Most of the problems we see are low body fat or over-exercising and getting infrequent periods or they lose them altogether," said Hoffman. "In the brain, the hypothalamus just goes ice cold. It's like a stress reaction -- fight or flight."